This paper examines the active mechanisms responsible for the size enlargement of nickel metal particles during reduction by hydrogen gas. The population balance, related to particle size distribution, is discretized and solved with the method of moments to model the experimental data. This data was generated in a solution of intermittently constant supersaturation during the batch reduction in an 11,000 1 of autoclave. PSD and mass distributions were determined by periodically removing suspension samples. In the proposed model, only the growth mechanism is taken into account and aggregation is ignored. Comparison of simulated and experimental PSD and mass distributions show that growth was not the only active mechanisms. Results from the plant indicate that there is an increase in particle size as the cycle progresses. The model generally underpredicts the growth shift in the PSDs' for the early and the larger densifications although it correctly predicts the growth shift in the middle densifications. Constant growth is therefore not the exclusive controlling mechanism. The findings have been confirmed by the trend of the zeroth moment as a function of time. (c) 2005 Elsevier B.V All rights reserved.